Flash memories and ferroelectric memories are known as non-volatile memories capable of retaining stored information even after the power is turned off.
Among these, flash memory has a floating gate embedded in a gate insulating film of an insulated-gate field-effect transistor (IGFET) to store information by accumulating, in this floating gate, electric charges indicating information. However, for such a flash memory, a tunnel current needs to be applied to the gate insulating film at the time of writing or deleting information. Thus, the flash memory is disadvantageous in that a relatively high voltage is needed.
In contrast, the ferroelectric memory, which is also referred to as FeRAMs (Ferroelectric Random Access Memories), store information by utilizing hysteresis characteristics of a ferroelectric film formed in a ferroelectric capacitor. The ferroelectric film is polarized in accordance with a voltage applied between the upper and lower electrodes of the capacitor, and spontaneous polarization remains even after the voltage is turned off. When polarity of an applied voltage is reversed, the spontaneous polarization is also reversed. Directions of the spontaneous polarization are associated with “1” and “0”, so that information is written in the ferroelectric film. Thus, FeRAMs are advantageous in that a voltage required for this writing is lower than that required in the case of flash memories, and that writing can be carried out at a higher speed than that in the case of flash memories.
FIG. 1 is a cross-sectional view of a capacitor Q of the FeRAM.
As shown in the figure, the capacitor Q is formed by stacking lower electrode 101, a capacitor dielectric film 102, and an upper electrode 103 in this order on an underlying film 100.
Among these, for the capacitor dielectric film 102, a PZT (Pb(Zrx, Ti1-x)O3) film is generally used. The ferroelectric characteristics, such as residual polarization charges, of the PZT film greatly depend on an orientation of PZT crystals. When the orientation of the PZT is aligned in (111) direction, its ferroelectric characteristic can be enhanced.
On the other hand, a stacked film including a titanium (Ti) film and a platinum (Pt) film formed in this order is used for the lower electrode 101. In this stacked film, titanium in the titanium film diffuses along the grain boundary of the platinum film, and then reaches a surface of the platinum film. When the PZT film is formed on the stacked film by a sputtering method, the titanium is oxidized by a small amount of oxygen contained in the PZT to form a titanium oxide (TiO2) nucleus. This titanium oxide serves as an initial growth nucleus of the PZT film, so that the orientation of the PZT film is aligned in the (111) direction.
Note that, the titanium oxide nucleus can also be formed when the PZT film is crystallized by an anneal in an oxygen atmosphere.
In addition, since the lattice mismatch of Pt (111) and PZT (111) is small, a PZT film with reduced defects due to the lattice mismatch can be formed on the platinum film.
The capacitor dielectric film 102 used in such a capacitor Q is required to have high-density crystals so that high ferroelectricity can be obtained even when the capacitor Q is miniaturized. Thus, in order to meet this requirement, a MOCVD (metal organic CVD) method is preferably employed as a method for forming the capacitor dielectric film 102, instead of a sol-gel method or a sputtering method.
However, if the PZT film is formed by the MOCVD method, lead (Pb) in the PZT film reacts with platinum in the lower electrode 101, and thereby surface roughness in the lower electrode 101 is caused. This surface roughness makes it difficult to align the orientation of the PZT film in the (111) direction.
In another method, an oxide, such as an iridium oxide (IrOx) film, is formed as the lower electrode 101, so that the PZT film is oriented in the (111) direction by the effect of the orientation of the oxide. However, when a PZT film is formed by the MOCVD method on the lower electrode 101 made of an oxide, the oxide is reduced by PZT, which results in causing the lower electrode 101 to be amorphous. This makes impossible to control the orientation of the PZT film by using the orientation of the lower electrode 101.
Accordingly, when the PZT film is formed by the MOCVD method, an iridium (Ir) film is often formed as the lower electrode 101. In this case, in order to form titanium oxide which is to be an initial growth nucleus of the PZT on the lower electrode 101, a titanium film may be formed under the iridium film to cause titanium to be diffused to the upper surface of the iridium film along the grain boundary of iridium.
However, since the iridium film has grains which are smaller and denser than those of the platinum film, the diffusion of titanium along the grain boundary of iridium cannot be expected, and thus the above-described initial growth nucleus is not generated. Hence, if a stacked film of an iridium film and a titanium film is used for the lower electrode 101, it is difficult to cause PZT to be oriented in the (111) direction by utilizing the initial growth nucleus of titanium oxide.
Moreover, the lattice constant of Ir (111) is smaller than that of PZT (111), so that the lattice mismatch of the iridium film and the PZT film is large. As a result, the PZT film formed on the iridium film is oriented in a direction different from a polarity direction (111), or is randomly oriented.
In the above description, the case of using titanium oxide as the initial growth nucleus of PZT has been described. In the Patent document 1 below, however, PbTiO3 is used as the initial growth nucleus.
However, since PbTiO3 is a ternary compound, it is difficult to control its composition ratio.
Patent document 2 discloses that a titanium oxide film is formed on an iridium film to be a lower electrode, and then the titanium oxide film is used as a nucleus to form a PZT film.
However, titanium oxide, such as TiO2, can be in various bonding states depending on an oxidizing temperature or atmosphere. Thus, it is difficult to control the orientation of titanium oxide.
In this manner, it has been extremely difficult to form, on an iridium film, a PZT film with an orientation aligned in the (111) direction.
In addition to the above-described techniques, Patent documents 3 and 4 also disclose techniques relating to the present application.    Patent document 1: Japanese Patent Laid Open 2000-58525    Patent document 2: Japanese Patent Laid Open Hei 10-12832    Patent document 3: Japanese Patent Laid Open Hei 9-282943    Patent document 4: Japanese Patent Laid Open Hei 11-297966